Sport-related concussion occurs when linear and angular head motion caused by external physical forces disrupts an athlete's brain function at the neuronal level. Athletes with smaller, weaker necks, including child and female athletes, are thought to be at increased risk of concussion because their necks are less able to counter these externally acting forces resulting in more violent head movement patterns. The bi-directional whiplash motion that occurs when an athlete's head rapidly changes its direction of rotation may also increase the risk of concussion associated with an individual impact, as compared to a uni-directional rotation pattern of similar magnitude. The proposed biomechanical investigation will pursue the following specific aims: AIM 1) to determine the relationship between sonographic measures of cervical muscle size and stiffness and the head's kinematic response under a standardized impulsive force in each anatomical plane across the spectra of age and gender; AIM 2) to determine whether in vivo commercial impact sensing systems can predict bi-directional versus uni-directional rotation patterns, as determined by a high speed kinematic motion capture system, during a sport-simulated bracing task. The working hypotheses associated with these aims are: H1) the magnitudes of the head's linear and angular velocity changes will be negatively associated with cervical muscle size and stiffness and will be more strongly correlated with these sonographic measures than concurrently measured neck strength in each plane (AIM 1); H2) resultant impact vectors generated by commercial impact sensing systems that are located inferior of the head's center of mass will be associated with bi-directional head rotation patterns, while impact vector locations superior to the head's center of mass will be associated with uni-directional rotation (AIM 2). The Candidate has a clinical background in Physical Medicine and Rehabilitation with expertise in mild traumatic brain injury, specifically sport-related concussion. He has master's level training in clinical research design and statistical analysis as well as additional basic training and experience in biomechanical engineering principles and laboratory techniques. He is committed to a career in sport concussion research focusing on injury biomechanics and he plans to use the additional training he receives through this NIH Career Development Award to support his future work in this field. The Candidate and his mentoring team have designed a career development plan to accomplish three training goals: 1) become proficient in biomechanical modeling techniques to permit effective use and interpretation of basic biomechanical models of the human head and neck, 2) obtain expertise in musculoskeletal ultrasonography, allowing for proficient measurement of cervical muscle size (cross sectional area) and stiffness (elastography), 3) improve grantsmanship skills to facilitate ongoing research support.